The titanium oxide film-forming methods known in the art may include, for example, (1) a method of coating a titanium oxide sol onto a substrate, followed by sintering, (2) a method of coating an aqueous solution of titanium chloride or titanium sulfate onto a substrate, followed by heat treating, (3) a plasma spray coating method which comprises fusing titanium oxide solid particles in the presence of plasma generated in atmosphere, followed by spraying onto a substrate, (4) a spatter spraying method of spattering the oxide as a target under vacuum, followed by spraying onto a substrate, (5) a chemical vapor deposition method of volatilizing an organic metal compound or the like, followed by decomposing in an electric furnace, and forming a film onto a substrate, (6) a sol-gel method of coating a sol obtained by hydrolysis of a metal alkoxide onto a substrate, followed by sintering, and the like.
However, the above methods (1) to (6) have such drawbacks that the method (1) is such that a film thickness of 0.1 μm or more may develop cracks and peelings, resulting in showing poor film-forming properties, and sintering at a temperature of hundreds is required, resulting in requiring much time and labor, the method (2) is such that adverse effects of thermal decomposition products on the substrate and necessity of sintering at a temperature of hundreds require much time and labor, the method (3) is unable to form a fine film and results poor adhesion properties, the methods (4) and (5) are such that a satisfactory film may be obtained under vacuum, so that a vacuumizing reactor may be required, and that a film-forming speed may be so low that heating the substrate at a temperature of hundreds to obtain a fine film, and the method (6) is such that the acid, alkali and organic compounds contained in the sol of the method (6) may produce problems of corrosion of a substrate, and that removal of the organic compounds requires heating at 400° C. or higher.
In addition to the above methods, the methods free of the step of sintering at high temperatures may include (7) a method of forming a titanium oxide film from an aqueous fluoride solution of titanium oxide and boric acid, (8) a method which comprises precipitating a titanium hydroxide gel from an aqueous solution of titanium chloride or titanium sulfate and an alkali solution such as ammonia, caustic soda and the like, followed by separating the titanium hydroxide gel by decantation, thoroughly washing with water, and adding hydrogen peroxide water (see Japanese Patent Application Laid-Open No. 71418/97), and the like.
However, the method (7) has such drawbacks that formation of the film requires much time, and handling of the fluoride is troublesome, and the method (8) has such drawbacks that the method (8) comprises complicated steps, removal of the alkali salt is troublesome, inclusion of other metals may make it difficult to obtain a high purity film, difficulty in control of pH results films different from each other depending on variation of pH, and so forth.